One known type of drive force transmission device, which transmits a drive force (a torque) from a driving-side rotatable body to a driven-side rotatable body through rotation, has a function of a damper for absorbing a torque fluctuation or shock between the driving-side rotatable body and the driven-side rotatable body (see, for example, JP2005-098448A).
This torque transmission device includes a rubbery elastic body (serving as a shock-absorbable body), which is configured into a circular ring form and is made of a rubbery elastic material, and the rubbery elastic body is placed between the driving-side rotatable body and the driven-side rotatable body.
The driving-side rotatable body includes a plurality of driving-side projections, and the driven-side rotatable body includes a plurality of driven-side projections. The driving-side projections and the driven-side projections are alternately arranged one after another about the rotational axis in the circumferential direction.
The rubbery elastic body includes a plurality of primary damper portions, each of which is inserted between a corresponding one of the driving-side projections and a corresponding one of the driven-side projections, and a plurality of secondary damper portions, each of which is inserted between a corresponding one of the driving-side projections and a corresponding one of the driven-side projections. Furthermore, each of a plurality of primary bridge portions connects between a corresponding one of the primary damper portions and a corresponding one of the secondary damper portions. Furthermore, each of a plurality of secondary bridge portions connects between a corresponding one of the primary damper portions and a corresponding one of the secondary damper portions.
The torque transmission device transmits the torque from the driving-side rotatable body to the driven-side rotatable body while each primary damper portion or each secondary damper portion is compressed and deformed between the corresponding driving-side projection of the driving-side rotatable body and the corresponding driven-side projection of the driven-side rotatable body.
Furthermore, the torque transmission device uses the rubbery elastic body. In this rubbery elastic body, each primary engaging groove, into which the corresponding driving-side projection is inserted, is defined by the corresponding primary bridge portion, the corresponding primary damper portion, and the corresponding secondary damper portion, and each secondary engaging groove, into which the corresponding driven-side projection is inserted, is defined by the corresponding secondary bridge portion, the corresponding secondary damper portion, and the corresponding primary damper portion. In the rubbery elastic body, two primary recesses are formed on two circumferentially opposite sides of each primary bridge portion, and two secondary recesses are formed on two circumferentially opposite sides of each secondary bridge portion.
By forming the two primary recesses on the two circumferentially opposite sides of each primary bridge portion and the two secondary recesses on the two circumferentially opposite sides of each secondary bridge portion, formation of a crack in the rubbery elastic body by repeatedly applied stresses (compression stresses), which are applied to the rubbery elastic body, is limited.
However, in the prior art drive force transmission device, the primary and secondary recesses are provided to the primary and secondary bridge portions to alleviate the stress generated in the primary and secondary engaging grooves in the rubbery elastic body when the rubbery elastic body is twisted between the driving-side rotatable body and the driven-side rotatable body in response to the torque transmitted from the driving-side rotatable body to the driven-side rotatable body. However, in a case where the torque, which is transmitted from the driving-side rotatable body to the driven-side rotatable body, is relatively large, a shock absorbing performance of the primary and secondary damper portions may not be sufficient, and thereby a crack may possibly be formed at the primary and secondary engaging grooves of the rubbery elastic body.
Particularly, in the case where the torque, which is transmitted from the driving-side rotatable body to the driven-side rotatable body, is relatively large, or in the case where a shock load is applied to the driving-side rotatable body or the driven-side rotatable body, a tensile stress may be applied to the primary and secondary bridge portions in response to the compression deformation at the primary and secondary damper portions of the rubbery elastic body. At this time, in the prior art drive force transmission device, a sufficient tensile elongation enabling part, which enables tensile elongation of the bridge portion, cannot be provided in the primary and secondary bridge portions due to the above described structure of the primary and secondary bridge portions.
There is a possibility of that a tensile stress is exerted at one of the primary and secondary bridge portions upon application of the compression stress to the one of the primary and secondary bridge portions. In such a case, a crack may be formed from one of the primary and secondary engaging grooves located adjacent to a bottom of one of the primary and secondary recesses at a circumferential side surface adjacent to one of the adjacent primary and secondary damper portions toward a center of the other one the adjacent primary and secondary damper portions to possibly cause severing of the other one of the adjacent primary and secondary damper portions.
Thus, the shock absorbing performance of the primary and secondary damper portions of the rubbery elastic body, or the durability of the rubbery elastic body may be disadvantageously deteriorated.